In the manufacture of metal containers, a protective organic coating is applied to the side of the metal sheet which is to form the interior of the container. The materials which are employed for coating the metal generally are heat curable resinous materials which are applied as a solution or dispersion in a volatile organic solvent. The coated metal surface is passed through an oven in which hot air is circulated to evaporate the solvent and to cure the coating film to the desired thermoset hardness.
This procedure is relatively slow, and there are the attendant disadvantages of troublesome odor, fire hazard and health threat, and high energy requirements.
A number of other coating compositions are known which are free of a volatile solvent, but which exhibit other properties which are undesirable and disadvantageous. A plastisol, such as polyvinyl chloride, provide coatings which are insufficiently resistant to the action of chemicals and solvents. Epoxy systems, which can be employed without a solvent, have numerous disadvantages. In addition to the health dangers and troublesome odors caused by the curing agents employed, epoxy systems have the additional disadvantage of a short processing time of coating mixtures, which causes handling difficulties and necessitates the use of two-component spray guns.
Similar disadvantages are exhibited by unsaturated polyester resins, such as short processing cycles and air-sensitivity of the curing reaction. The polyurethane coating systems have the disadvantage of being a health hazard because of the isocyanate content. Also, the systems suffer from high sensitivity to water, which results in the formation of faults and bubbles in the coatings if water is not rigorously excluded.
For the foregoing reasons, research effort has been dedicated to the development of aminoplast-based solvent-free coating systems. Such aminoplast coatings are often characteristically brittle and inelastic, and may exhibit other undesirable properties such as poor adherence to metals and unsatisfactory mechanical properties. Also, aminoplast resin blends with conventional varnish resins have the disadvantage of high viscosity, and in many cases, limited compatibility between the resin components.
Accordingly, it is a main object of the present invention to provide a low viscosity coating composition which cures as a substantially 100 percent solids thermoset film exhibiting a desirable combination of hardness and flexibility.
It is another object of this invention to provide an aminoplast coating system which cures rapidly to yield protective films with excellent adherence to metal substrates, and which exhibit a desirable combination of hardness and elasticity.
It is a further object of this invention to provide an acid-hardening condensation polymerizable aminoplast/alkyd resin composition which has a thermal stability gel time of at least 10 days at a temperature of 120.degree. F.
Other objects and advantages shall become apparent from the accompanying description and examples.